1. Field of the Invention
The present invention relates to an electromagnetic wave generator for generating electromagnetic waves such as X-rays, by means of electrons that, within an accelerator, circulate while forming a circular orbit.
2. Description of the Related Art
Conventional electromagnetic wave generators utilizing a circular accelerator include a generator (Non-Patent Literature 1) utilizing an accelerator (shortly referred to as a betatron accelerator) based on the betatron acceleration principle and a generator (Patent Literature 1) utilizing an electron storage ring.
In an electromagnetic wave generator utilizing a betatron accelerator, electrons injected into the generator are accelerated, while circulating in an orbit of a constant radius; when their energy have reached a predetermined level, the electrons are made to change its orbit, whereby the electrons collide with a target arranged in the resultant orbit, thereby generating X-rays (Non-Patent Literature 1).
In addition, an electromagnetic wave generator utilizing an electron storage ring is configured of an injector and the electron storage ring; electrons that have been accelerated so as to have predetermined energy are injected from the injector into the electron storage ring and circulate along constant orbits within the ring. In the closed orbit, a target is arranged; the collision between the target and the circulating electron beam generates X-rays (Patent Literature 1).
[Non-Patent Literature 1] Accelerator Science (Parity Physics Course) co-authored by Toru Kamei and Motohiro Kihara, published by Maruzen Co., Ltd., on Sep. 20th, 1993 (ISBN 4-621-03873-7 C3342), Chapter 4 “Betatron”, 39p-43p [Patent Literature 1] Japanese Patent No. 2796071
The foregoing electromagnetic wave generators have the problems described below. In an electromagnetic wave generator utilizing a betatron accelerator (Non-Patent Literature 1), due to coulomb repulsion between electrons that circulate within the accelerator, high-current acceleration is difficult to implement. Accordingly, compared with an electromagnetic wave generator utilizing a linear accelerator, the intensity of accelerated electrons are small by approximately one or two digits. In addition, in this type of accelerator, an electron dosed orbit is maintained constant while an electron beam is accelerated so as to have predetermined energy. Accordingly, in order to make the beam collide with a target, its orbit is required to be shifted to the orbit in which the target for generating X-rays is arranged. However, the beam off the closed orbit cannot stably circulate, whereby it is difficult for the beam to collide with the target repeatedly. For that reason, the intensity of generated X-rays is low; therefore, it has been almost impossible that an electromagnetic wave generator utilizing a betatron accelerator is applied to the industrial or the medical field.
In addition, in order to obtain X-rays having different energy levels, the energy of electrons that are made to collide with a target is required to be changed; however, in a betatron accelerator, an electron beam whose orbit has been changed to another orbit in which the electron beam collides with the target cannot stably circulate, whereby the electron beam disappears. Accordingly, in order to generate the next X-rays, injection and acceleration are required to be resumed; therefore, it has been impossible to generate X-rays having different energy levels, in a high-speed switching fashion. Furthermore, because the consistency in the respective positions of injected electron beams is not necessarily accurate, the position where the electron beam collides with the target may subtly be shifted from one another. Accordingly, the precise measurement, through the high-speed energy subtraction method, on a movable subject has been difficult due to problems in high-speed switching of X-ray energy and in consistency in the respective X-ray-source positions for electron beam injections. Moreover, when, even in the case where the high speed is not required, measurement is implemented through the energy subtraction method, a subtle positional shift of an electron beam that collides with the target causes a positional shift of an X-ray source, whereby it has been difficult to implement precise measurement.
In an electromagnetic wave generator utilizing an electron storage ring (Patent Literature 1), the closed orbit of an electron beam is basically constant; therefore, it is possible to make the electron beam recurrently collide with the target, whereby the X-ray intensity is improved, compared with a betatron accelerator. However, in an electromagnetic wave generator utilizing an electron storage ring, it is difficult to make the value of the injection current large, and an injector and an electron storage ring for accelerating electrons so as to have predetermined energy, whereby the generator becomes large-scale; therefore, the number of constituent apparatuses increases and control is rendered complicated. As a result, the electromagnetic wave generator has been high-cost and its application fields have been limited.
Even though having a function of maintaining the energy of circulating electrons at a predetermined value, the storage ring does not have a function of varying the energy; in order to vary the energy, it is necessary to vary in the injector the injection energy of the electrons to be injected into the storage ring. Accordingly, also in this case, as is the case with a betatron accelerator, it is difficult to generate X-rays having different energy levels, in a high-speed switching fashion; therefore, as is the case with a betatron accelerator, the application fields of the electromagnetic wave generator utilizing an electron storage ring is limited. In addition, if the storage ring is provided with an acceleration function and utilized as a synchrotron accelerator, it is possible to vary the energy of an electron beam that is already circulating within the accelerator; however, it is difficult to ensure the high-speed energy switching, and a further problem is that, in that accelerator, the closed orbit of an electron beam is constant even during the acceleration, whereby, during the acceleration, the target has to be arranged off the closed orbit so that the collision between the electron beam and the target should be avoided. In this case, after colliding with the target, the circulating electron beam cannot stably circulate; therefore, as is the case with a betatron accelerator, it is difficult for the electron beam to collide with the target repeatedly.